The Ca2+/calmodulin-dependent protein kinase II (CaMKII) is required for forms of learning and memory. In brain, CaMKII is expressed at levels reminiscent of structural proteins (1-2% of total protein), and its 12meric holoenzyme structure allows multiple simultaneous protein interactions. However, structural roles of CaMKII or functions of its interactions with other protein are understood poorly at best. The major excitatory neuro- transmitter in mammalian brain, glutamate, induces two types of CaMKII translocation in hippocampal neurons: To postsynaptic sites upon brief stimulation (0.5-2 min) and to extra-synaptic clusters after extended exposure (3-5 min). Both types of translocation require the multivalent CaMKII holoenzyme structure (unpublished observations). Brief glutamate treatment can induce forms of synaptic plasticity thought to underlie learning and memory, while extended exposure leads to excitotoxicity, a cellular model for ischemic cell death. The goal of this proposal is to test our hypotheses that synaptic translocation requires binding to the NMDA-type glutamate receptor subunit NR2B and is involved in synaptic plasticity, while formation of extrasynaptic clusters is mediated by CaMKII self-association and is involved in ischemic cell death. Elucidating the mechanisms and functions of CaMKII targeting will aid our understanding of synaptic principles underlying higher brain functions and behavior, and provide new therapeutic avenues for ischemic damage. We will accomplish our goal in the following aims (using rat hippocampal and cortical cultures as main model systems for studies in neurons): (1) Determine protein interactions required for subcellular CaMKII localization in neurons. We will identify CaMKII mutations and inhibitors that prevent NR2B-binding or self-association in vitro and determine their effect on GFP-CaMKII localization in neurons. (2) Determine neuroprotective versus neurotoxic effects of CaMKII activity, synaptic targeting, and extrasynaptic clustering. We will make use of an inhibitor of CaMKII activity and clustering, knockdown of expression by RNAi, and overexpressionof CaMKII wildtype and mutants with specific impairments in activity, regulation and targeting. (3) Determine functions of CaMKII in regulation of synapse number and strength. We will make use of the same tools as in (2) and determine effects on synapse number and strength in established assays.